When the world's first international urban-planning conference was held in New York in 1898, it was dominated by a seemingly unsolveable transportation issue that seemed to threaten city life. Finding no solution to the problem, the conference broke up a week early. What was the problem?
The answer was horse manure, which had become so abundant farmers were no longer hauling enough of it away for fertilizer, so it got stacked in vacant lots. This problem cleared up in subsequent years due to the rise in the use of the automobile and the subway. J. Mladek was the first with the right answer. If I can get a t-shirt size and an address, I can send out the prize.